In a cargo ship transporting crude oil, containers and the like, a ballast tank is provided in order to maintain stability of a hull during navigation. Usually, if crude oil and the like are not loaded, the ballast tank is filled with ballast water, and the ballast water is discharged when crude oil, containers and the like are loaded so as to adjust a buoyancy of the hull and stabilize the hull.
As mentioned above, the ballast water is water required for safe navigation of a ship, and seawater at a port where cargo handling is carried out is usually used. Its amount is estimated to be 3 to 4 billion tons on the worldwide basis.
In the ballast water, aquatic organisms living in a port from which the water was taken are mixed, and the aquatic organisms are also transported to foreign countries with movement of the ship.
Therefore, destruction of an ecological system, in that a species foreign to a local sea may replace an indigenous species, is a serious problem.
With such a background, at a diplomatic conference of the International Maritime Organization (IMO), the International Convention for the Control and Management of Ships' Ballast Water and Sediments (hereinafter referred to as the convention) was adopted, and a duty to execute ballast water management using a ballast water treatment apparatus is scheduled to be applied to ships constructed in 2009 and after.
Also, a ballast-water discharge standard is specified by the convention as shown in Table 1 below:
TABLE 1Ballast waterItemquality standardSizeAquatic organism10pieces/ml10 to 50 μmAquatic organism10pieces/m350 μm or moreIndicatorEscherichia Coli250cfu/100 mlbacteriaPathogenic cholera1cfu/100 mlvibrio (01, 0139)Enterococcus100cfu/100 ml
From the above background, development of a ballast-water treatment technology that can solve the above problems is urgently needed.
In the past, as a method of physically treating water containing aquatic organisms such as plankton, a technology to disinfect or sterilize the aquatic organisms in the ballast water by injecting an ozone gas into the ballast water is disclosed in Patent Document 1.
Also, a technology to mechanically destroy and kill the aquatic organisms by having water containing the aquatic organisms pass a slit plate by a high-pressure pump is disclosed in Patent Document 2.
Currently, in a ship navigating the ocean, a ballast pump for taking in the ballast water and a ballast-water system piping for supplying the ballast water into a ballast tank are provided. As an existing ballast pump, a high flow-rate/low-pressure pump with a specification of 600 m3/h, 0.25 MPa as an example is used.
The inventors tried to apply the slit plate described in Patent Document 2 to the ballast-water system piping, but it was found out that the existing ballast pump lacks in pressure and the slit plate does not function. Reduction of a treatment water amount can be one solution, but time constraints apply to supply of the ballast water into the ballast tank (such as constraint of time allowed for anchorage, for example), and it is difficult to extremely reduce the treatment water amount.
Also, in the existing ballast pump, pipings and the like for taking in the ballast water and for discharging the ballast water are installed in a complicated manner, and if the treatment water amount is reduced, the diameter of the piping should be also changed, and if the flow rate is to be changed by a valve, it is a difficult work to install the valve in a narrow space, and this solution is costly.
Then, the inventors tried to develop a practical technology to efficiently treat the ballast water with a required minimum remodeling effectively using the existing ballast pump or existing ballast-water system pipings and have realized the present invention.    Patent Document 1: Japanese Patent Laid-Open No. 2004-160437    Patent Document 2: Japanese Patent Laid-Open No. 2003-200156